The manufacture of blocked or masked polyisocyanates is known and is described in Houben-Weyl "Methoden der Organischen Chemie" ("Methods of Organic Chemistry"), volume 14/2, pages 61-70.
Reaction products of hexamethylene-1,6-diisocyanate with tert.-butanol, phenol, acetoacetic acid ethyl ester, malonic acid ester, acetylacetone, phthalimide, imidazole, hydrogen chloride, hydrogen cyanide and caprolactam are known.
This aliphatic diisocyanate was reacted with compounds which are split off again at elevated temperature to liberate the isocyanate group. Products of this type are designated isocyanate donors or "moderators". In contrast with the free diisocyanates, masked diisocyanates of this type make it possible to manufacture mixtures with substances or solvents which contain hydroxyl groups, without a reaction taking place in the course thereof. It is therefore possible, by means of masked polyisocyanates, to manufacture mixtures with products which contain hydroxyl groups, such as higher molecular polyesters or polyethers, and which are stable on storage and only give the desired isocyanate reactions at an elevated temperature. They are of great importance both for the manufacture of rubber-elastic products via storable intermediate stages and for the manufacture of wire lacquers, and in the textile field. The donor effect comes about because virtually all adducts which are formed from isocyanates at a moderately elevated temperature redissociate once more at higher temperatures, equilibria being set up. The establishment of these equilibria is accelerated by adding tertiary bases.
In combination with polymers containing hydroxyl groups, many of these masked polyisocyanates exhibit an unsatisfactory crosslinking at low stoving temperatures. Other masked polyisocyanates which redissociate at lower temperatures produce scission products which must not be employed for stoving lacquerings because of their toxicity.
A known adduct, manufactured from hexamethylene diisocyanate and acetoacetic acid ethyl ester, produces two scission products, namely acetoacetic acid ethyl ester, which is less toxic, and hexamethylene diisocyanate, which is not physiologically harmless. This toxic effect is further increased because the hexamethylene diisocyanate is only liberated above 140.degree. C. The known adduct has a melting point of 81.degree.-82.degree. C. and must therefore be manufactured in the melt above the melting point temperature, that is to say at about 90.degree. C., as a result of which an undesirable yellow coloration takes place which can only be removed by recrystallisation. This solid adduct has a tendency to crystallise and leads to an undesirable inhomogeneity in the manufactured lacquer solutions. Thus, for example, stoved lacquers with greatly impaired film properties are produced by the combination of polymers containing hydroxyl groups and the known adduct.
It is an object of the invention to provide completely or partially blocked polyisocyanates which are present, at room temperature and also at temperatures of about 0.degree. C., as solutions in inert solvents which are customary in the lacquer industry.
It is a further object of this invention to provide completely or partially blocked polyisocyanates which, when completely blocked, produce, in the temperature range from 80.degree. to 130.degree. C., preferably 90.degree. to 110.degree. C., crosslinked reaction products in the presence of polymers containing hydroxyl groups. In addition, the physiological behaviour of the blocked polyisocyanates, also after their redissociation into the starting components, should be less harmful than that of the known blocked and free polyisocyanates.